1. Field of The Invention
The present invention relates generally to radiation therapy. More specifically, the present invention relates to a system, program product, and related methods for determining radiation dose to be delivered according to a radiation treatment plan.
2. Description of the Related Art
Radiation therapy can be effective in treating certain types of cancerous tumors, lesions, or other “targets.” A vast majority of such targets can be eradicated completely if a sufficient radiation dose is delivered to the tumor or lesion volume. High-energy radiation is absorbed and scattered by matter. Cancer cells forming the tumors are often more sensitive to radiation than normal cells, so radiation treatment is often used to fight cancerous tumors. Those tumors are usually deep inside the body, and when radiation coming from an external source is applied, it is inevitable that normal surrounding tissue will receive radiation. The objective is thus to give the tumor a lethal amount of radiation while keeping under acceptable levels the amount of radiation that healthy tissue will receive. For most of the cases high energy photons and electrons are employed for treatment, but protons, neutrons, heavy charged particles, etc, are also used. Complications, however, may result from use of the necessary effective radiation dose due to damage to healthy tissue which surrounds the target or to other healthy body organs located close to the target. The goal of the various radiation procedures, such as conformal radiation therapy treatment, nevertheless, is to confine the delivered radiation dose to only the target volume defined by the outer surfaces of the target, while minimizing the dose of radiation to surrounding healthy tissue or adjacent healthy organs. If the effective radiation dose is not delivered to the proper location within the patient, serious complications may result.
Radiation treatment therapy delivery typically uses a radiation delivery apparatus, such as, for example, a linear accelerator or other radiation producing source, to treat the target. The conventional linear accelerator includes a rotating gantry which generally rotates about a horizontal axis and which has a radiation beam source positionable about the patient which can direct a radiation beam toward the target to be treated. The linear accelerator can also include a rotating treatment table which generally rotates about a vertical axis and which can position the target within a rotational plane of the rotating gantry. Various types of devices or apparatus can set the field size to further conform the shape of the radiation treatment beam during rotation of the radiation beam source to follow the spatial contour of the target, as viewed with respect to the radiation treatment beam, as it passes through the patient's body into the target. The modern radiation sources, such as the linear accelerator, have primary collimators (jaws) that set the field size. Often they are also equipped with special collimators, e.g., multi-leaf collimators (MLC), which have multiple leaf or finger projections that can be programmed to move individually into and out of the path of the radiation beam to shape the radiation beam to dynamically shape the field of irradiation in order to deliver dose in the desired way.
Typically, the patient has the radiation therapy treatment plan prepared based upon a diagnostic study utilizing computerized tomographic (“CT”) scanning, magnetic resonance (“MR”) imaging, or conventional simulation films which are plain x-rays generated with the patient. This radiation therapy treatment plan is developed such that the patient's tumor or lesion is in the position that will be used during the radiation therapy treatment. Various types of radiation treatment planning systems can be used to create the radiation treatment plan which, when implemented, will deliver a specified dose of radiation shaped to conform to the target volume, while limiting the radiation dose delivered to sensitive surrounding healthy tissue or adjacent healthy organs or structures. Various forms of radiation treatment planning include forward planning and inverse planning. In forward planning the physicist directly controls the machine settings of the beams by manually setting the shape and radiation dose of each field utilizing knowledge of a past treatments in order to achieve expectations of the physician. With inverse planning the physician directly prescribes the desired target dose and tolerances for sensitive structures, and the optimization software determines machine settings that will most closely deliver the prescribed radiation distribution. In the case of both forward planning and inverse planning, a procedure is required to calculate the radiation dose associated with the machine settings of the beam. In inverse planning, the optimization software explores a multitude of possibilities for the beam settings so that computational complexity (calculation time) is critical. To this end, the radiation beam field can be partitioned into many small rectangular or square shaped fields which are generically called finite-size pencil beams (FSPB) or pencil beams, for short. That is, a large radiation beam field can be composed of many pencil beams. The FSPBs allow for optimal partitioning of the radiation field and they are computationally efficient for calculating dose distributions of complex modulated fields. In the intensity modulated radiation therapy (IMRT), once the parameters for the pencil beams are computed, their intensities are modulated until the optimal dose distribution is achieved. From a computational point of view, FSPB dose values can be stored in tables and a table lookup method can be used.
Most current methods used to calculate the dose delivered to the target volume and surrounding structure are based on dose measurements made in a water box. Heterogeneities such as bone and airways are treated in an approximate way or otherwise ignored altogether. Next to direct measurements, the most accurate way of calculating dose in a heterogeneous medium is employing the Monte Carlo (MC) method. Superposition/convolution is a close alternative. Hundreds or even thousands of pencil beams need to be pre-computed for a regular treatment plan. Traditional Monte Carlo and superposition/convolution algorithms require computing the dose distribution for entire volume in order to determine dose a single point of interest. Thus, both algorithms are computationally very expensive. Monte Carlo requires simulating tens of millions of particles through the whole volume to calculate radiation dose at the point of interest. Superposition/convolution requires completion of a 3D convolution to calculate radiation dose at a point of interest. Due to the enormous amount of point dose calculations required to optimize a plan, use of the Monte Carlo method, without modification, will remain impractical for inverse planning.
A. Van Esch, et al, in an article titled “Testing Of The Analytical Anisotropic Algorithm For Photon Dose Calculation,” Med. Phys. 33, 4130 (2006), describes an algorithm known as the Varian AAA inhomogeneity algorithm, which calculates “photon dose . . . as a three-dimensional convolution of Monte-Carlo precalculated scatter kernels, . . . .” Cormen et al. in, e.g., “Introduction to Algorithms”, The MIT Press, Cambridge Mass. (1997), however, indicates that a convolution, most efficiently implemented through the Fast Fourier transform, is known to have computational complexity O(n lg n), where “n” is the size of the vector being convolved and ‘lg’ represents a logarithm with undisclosed base. An alternative method of computing primary central axis dose is based upon convolution with a forward and backward spread function rather than a finite difference equation, described, e.g., in a publication titled “A Method Of Calculating High-Energy Photon Primary Absorbed Dose In Water Using Forward And Backward Spread Dose-Distribution Functions,” Med. Phys. 12, 731 (1985), again, is a non-constant time operation.
Monte-Carlo codes such as PEREGRINE®, described, e.g., in C. Hartmann, et al, “Description and Dosimetric Verification of the PEREGRINE® Monte Carlo Dose Calculation System for Photon Beams Incident on a Water Phantom,” Med. Phys. 28, 1322 (2001), require a full simulation to determine dose at a single point, and thus, cannot determine dose to a single point in constant time. Likewise, even a fast variant of superposition convolution, such as, for example, the Collapsed Cone method, described, e.g., by A. Ahnesjo, in a publication titled “Collapsed Cone Convolution of Radiant Energy for Photon Dose Calculation in Heterogeneous Media,” Med. Phys. 16, 577 (1989), require a full simulation to determine dose at a single point. Similarly, direct application of a Clarkson Integration for inhomogeneous media, sector integration is required for each point of interest; integration also being a non-constant time operation.
Accordingly, neither the Monte Carlo nor superposition/convolution methods can compute dose to a point with constant time computational complexity. Rather, computing dose to a single point requires simulation of the energy transport through the entire spatial distribution of electron densities. As a result, computing dose to a small subset of points in a volume essentially requires calculating dose to the whole volume. IMRT optimization requires rapid exploration of a multitude of candidate treatment plan solutions to some points of interest. Full simulation cannot be employed for each of the multitude of candidates. Interactive manipulation of radiation dose distributions as in U.S. Patent Application 20050111621 requires rapid calculation of a few high-resolution dose images which are beyond the capabilities of the transport simulating algorithms.
The traditional pencil-beam method was developed to provide dose computations to a point with constant time computational complexity. This method, however, has significant inaccuracies in regions of lateral disequilibrium such as for a narrow beam passing through the lung or other region of electron density below that of water as in Nizin, “Electronic Equilibrium and Primary Dose in Collimated Photon Beams,” Med. Phys. 20, p. 258 (1982). Lateral disequilibrium is an effect of electron scattering: when the beam is small or energy is high in low-density media, such as lung material, the traditional model will systematically overestimate the central axis dose and underestimate the width of the beam. This method also has significant inaccuracies in media having lateral heterogeneities whereby the beam experiences a variation in electron density across the beam front at a given depth. These inaccuracies typically separately result in an overestimate of central axis dose in the lower electron density portion. Further, this method has significant inaccuracies in media having a complex electron density distribution such as the human body because it does not adequately account for multiple build-up and build-down regions characteristic of media having a complex electron density distribution. Rather, this method employs a single dose build-up restriction provided to model initial dose entry into the media. For complex media, such restriction typically results in an overestimate of central axis dose in points or regions having an electron density other than that of water, particularly with respect to narrow or high-energy beam fields.
There have been efforts to improve the results for when the traditional pencil-beam method is used in heterogeneous media. Many such efforts, however, assumed the beam was passing through a slab geometry phantom where the electron densities did not vary in a fully three-dimensional manner. For example, one traditional method of accounting for heterogeneities called the effective path length method (EPL) amounts to substituting the integral of electron densities along a path for the depth. Such attempts to improve the traditional pencil beam method, however, only account for part of the effect of the heterogeneous media through an effective path length by adding up the electron densities at all the points between the skin and the depth of interest. Specifically, these attempts to improve the traditional pencil-beam method do not address the important effects of penumbra widening in the lung or other low density structure and the effects of lateral heterogeneities as the radiation beam passes through the complex electron density distribution of the human body. Nor do they address the effects of the complex electron distribution resulting in continuous density changes, and thus, continuous build-up/build-down.
A few research avenues are noted regarding Monte Carlo inverse planning which relate to the use of pencil beam algorithms. First, Monte Carlo can be employed in conjunction with a pencil-beam algorithm. In such case, Monte Carlo calculations are preformed on a few iterations using pencil-beam calculations in intermediate iterations as described in Siebers, et al, in “Performance of a hybrid MC dose algorithm for IMRT optimization dose evaluation,” Med. Phys. 34, 2853 (2007). Recognized by the Applicants is that numerous calculations would still be required to obtain dose at a single point of interest, and that improvements to the pencil-beam accuracy would be desirable. Second, Monte Carlo generated pencil-beams can be applied as described in Bergman et al., in “Direct Aperture Optimization for IMRT Using Monte Carlo Generated Beamlets,” Med. Phys. 33, 3666 (2006). Recognized by the Applicants is that the points of interest used for optimization must be pre-selected rather than arbitrarily placed, that calculating dose at a point of interest that was not preselected would require a complete Monte Carlo simulation, and that this avenue does not provide a system or a method of updating an arbitrary 2d image profiles in real-time. Accordingly, recognized by the Applicants is the need for a system and method which can generalize the Monte Carlo generated central axis data to off-axis profiles calculation, for example, to enable interactive 2d dose image calculations.
Regardless of which methodology is used at the time of a diagnostic study to develop the radiation therapy treatment plan, in the delivery of either conformal radiation therapy treatments or static radiation therapy treatments, an accurate and repeatable determination of radiation dose to the delivered is very important. Successful radiation therapy depends on accurately placing the proper amount of radiation upon the target without unnecessarily damaging surrounding tissue. Thus, it is necessary to relate the radiation dose determined to be delivered to the target at the time of the diagnostic study to the radiation dose actually delivered at the time of the radiation therapy treatment. If the actual dose is not correct, the result can be under-treating the target tumor or lesion and/or damaging healthy surrounding tissue and organs.
Recognized, therefore, by the Applicants is the need for a system, program product, and methods for determining and determining dose to be delivered to a patient that provides enhanced accuracy for real-time dose optimization, provides values at three-dimensional point without needing to determine values for the entire volume or subset thereof, that accounts for reduced actual dose and wider penumbra resulting from lateral electronic disequilibrium, that accounts for the complex electron density distribution of the human body, and that accounts for variations in electron density across the beam front or lateral heterogeneities, to thereby provide enhanced accuracy for determining dose in low-density, e.g., lung, material particularly when using narrow or high beam energy.